A turbine is a fluid flow machine, which converts the internal energy (enthalpy) of a flowing fluid (liquid or gas) into rotational energy and ultimately into mechanical drive energy. A proportion of the fluid stream's internal energy is removed therefrom by the maximally eddy-free, laminar flow around the turbine blades and passes to the rotor blades of the turbine. This then sets the turbine shaft in rotation, and the useful power is output to a coupled-on working machine, such as for example a generator. The rotor blades and shaft are parts of the mobile turbine rotor or wheel, which is arranged within a housing.
As a rule, a plurality of blades are mounted on the shaft. Rotor blades mounted in a plane in each case form a blade wheel or impeller. The blades have a slightly curved profile, similar to an aircraft wing. A stator is conventionally located upstream of each impeller. The stator guide vanes protrude from the housing into the flowing medium and cause it to swirl. The swirl (kinetic energy) produced in the stator is used in the subsequent impeller to set in rotation the shaft on which the impeller blades are mounted. The stator and impeller together are known as a stage. A plurality of such stages are often connected in series.
The rotor of a turbine is as a rule held together in the axial direction by means of a tie rod. The individual rotor components such as turbine wheel disks, rotor disks and hollow shafts are arranged in a row and clamped by a tie rod. The rotor disks are here connected together interlockingly by Hirth toothing, such that torque may be transferred between the individual elements.
To reduce oscillation of the tie rod, the tie rod is in this case held by bracing means which are inserted in the various compressor and turbine wheel disks and in the cooling air separation tube. To this end, annular, conically bevelled coupling elements are conventionally provided, which engage in a groove introduced into the respective rotor component, said groove extending in the circumferential direction and being open in the axial direction. The coupling elements are here heated on assembly, so that they are connected by shrink fit in the groove of the respective rotor component such as for example a wheel disk. Due to the conical shape, the coupling elements enclose the tie rod flush at their smallest diameter and likewise exhibit a shrink fit at this point.
However, with the known bracing means an additional axial securing component is typically necessary to prevent any possible axial travel. For example, the retaining elements must always be placed between two disks. Despite these measures, the risk of a temporary, transient loss of contact still exists.
It is moreover known from DE 2 135 088 A1 to secure the tie rod of a rotor of a fluid flow machine relative to an outer casing by way of a circumferentially toothed pair of bushes.
In addition, US 2007/0286733 A1 discloses thermal separation of rotor disks and tie rod of a gas turbine. To this end, an insulation ring and two spacer segments inserted from radially outside are arranged between the last rotor disk and the end of the tie rod. To secure the latter against loss caused by centrifugal force, a sleeve is put over the insulation ring and the spacer elements, which sleeve is in turn secured by a split ring against axial displacement. A disadvantage here is that the tie rod is not braced between its two ends and is thus capable of oscillating.